1. Field of the Invention
This invention relates to novel microporous asymmetric polyamide membranes. More particularly, this invention relates to novel integral gradient, submicroscopically porous, anisotropic, hydrophilic, polyamide membranes which are asymmetric in pore size, i.e., which have an increasing gradation in average pore diameter from one surface of the membrane to the other. These membranes are particularly useful as microfilters, and especially as the filtration components of microfiltration cartridges, which can be used to remove fine particle size contaminants having diameters as small as about 0.01 .mu.m from fluids such as those encountered in the electronics and pharmaceutical industries. For example, these novel membranes can be used in cold sterilization processes to remove bacterial contaminants, and to remove fine particle contaminants from fluids used in the manufacture of microchips. This invention also relates to a novel process for preparing these microporous asymmetric polyamide membranes.
2. Description of the Prior Art
Porous polyamide membranes or films, and various processes for their preparation, including both "dry" and "wet" processes, have been known for many years.
Microporous polyamide membranes characterized by being "skinless" and consequently unlike prior art skinned microporous polyamide membranes are disclosed in U.S. Pat. Nos. 4,340,379 and 4,340,380 to Pall. And, among the patents acknowledged by Pall as disclosing prior art skinned membranes are U.S. Pat. Nos. 3,615,024 to Michaels and 3,876,738 to Marinaccio et al.
In U.S. Pat. No. 4,595,503 to Schlindler et al, polyamide membranes are disclosed which are said to be suitable for use in ultrafiltration applications, i.e., filtration of high molecular weight (macromolecular) substances. These membranes are made up of an ultrafiltration skin formed by subjecting a membrane-forming polyamide solution in formic acid (in which the minimum concentration of formic acid is 75%), cast or spread onto a carrier film, to a dwell time of a few seconds in ambient air before immersion in a coagulating and washing bath, and a backing layer, i.e., that portion of the initially cast or spread membrane-forming polyamide solution which has not formed the ultrafiltration skin, whose pore size increases with increasing distance from the ultrafiltration skin.
The Schindler et al patent teaches that these membranes, after being coagulated and washed, must be stretched in at least one direction before they are dried to ". . . increase [their] strength and allow adjustment of the pore size".
The problem inherent in Schindler et al's membranes and the process by which they are prepared, especially if one were to attempt to practice this process commercially, is this stretching Schlinder et al's unsupported thin membranes could easily damage them, and give unacceptably large pores or tears on the backing layer side or even in the ultrafiltration skin.
Microporous polyamide "breathable" structures which are permeable to gases, including water vapor, prepared by distributing a layer of a polyamide resinous solution over a woven, knit or felted fabric, cooling the resulting composite and then removing the solvent are disclosed in U.S. Pat. No. 3,427,179 to Davis.
The above-mentioned Pall patents also disclose:
". . . [skinless] polyamide resin membranes that are supported by the substrate on which the polyamide resin membrane is formed, either imbedded therein, or having the substrate attached to one face thereof", or PA1 ". . . [skinless] microporous polyamide resin membrane composites having a plurality of polyamide resin membrane layers, formed of membranes prepared separately by precipitation on separate substrates and then bonded together by drying two or more layers maintained in close contact";
see, e.g., U.S. Pat. No. 4,340,479 at column 12, lines 5-15.
Skinned polyamide resin microporous asymmetric membranes, however, offer the following advantages over skinless polyamide resin membrane symmetric composites such as those disclosed in the Pall patents: (1) higher flux rates for a given porosity; (2) improved skin dynamics; (3) superior dirt holding capacity; and (4) longer service life.
There is a need for chemically and physiologically inert skinned microporous asymmetric polyamide membranes whose pore structures render them particularly suitable for microfiltration applications and whose method of preparation does not give rise to the likelihood that the membrane, or its pore structure, will be damaged during preparation.
It is an object of this invention to provide novel microporous asymmetric polyamide membranes.
It is also an object of this invention to provide novel microporous asymmetric polyamide membranes useful as filtration means.
It is a further object of this invention to provide novel integral gradient microporous polyamide membranes in which the pore sizes along the gradient may be precisely selected.
Another object of this invention is to provide novel integral gradient, skinned, submicroscopically porous, anisotropic, hydrophilic, asymmetric polyamide membranes, whose unique pore structure, high porosity, high strength, high flux and superior dirt holding capacity make them particularly useful as microfilters, and especially as the filtration components of microfiltration cartridges, which can be used to remove fine particle size contaminants from fluids such as those encountered in the electronics and pharmaceutical industries.
A still further object of this invention is to provide a novel process for preparing these microporous asymmetric polyamide membranes.
These and other objects, as well as the nature, scope and utilization of the invention, will become readily apparent to those skilled in the art from the following description, the drawings and the appended claims.